Heat treatment is applied to, for example, vehicle structures such as a center pillar and a reinforcement to ensure strength. Heat treatment can be classified into two types, namely, indirect heating and direct heating. An example of indirect heating is a furnace heating in which a workpiece is placed inside a furnace and the temperature of the furnace is controlled to heat the workpiece. Examples of direct heating include induction heating in which an eddy current is applied to a workpiece to heat the workpiece, and a direct resistance heating (also called as a direct electric conduction heating) in which an electric current is applied directly to a workpiece to heat the workpiece.
Some automotive parts are formed by pressing a tailored blank, which is made by, for example, welding plates made of different materials and/or having different thicknesses (see, e.g., JP2004-058082A).
When pressing such a tailored blank, only a portion of the tailored blank may be heated to a quenching temperature, without heating the non-quenching region of the tailored blank to the quenching temperature. To implement this heating, the respective heating temperature may be adjusted by controlling the amount of electric current applied to a pair of electrodes provided on the quenching region of the blank and the amount of electric current applied to another pair of electrodes provided on the non-quenching region of the blank, respectively.
That is, when heating a workpiece like a tailored blank to have a desired temperature distribution, a plurality of pairs of electrodes is provided for a single workpiece, and the amount of electric current applied is controlled for each pair of electrodes. This is undesirable from the viewpoint of facility cost.